There are actually two methods of doing it - stoichimetry & also with the help of law of conservation of mass.
First by :
Now, after balancing the entire equation we get:
So, 1 mole of methane produces 2 moles of water. Hence,
Now by law of conservation of mass:
The entire mass of the reactants will be equal to the entire mass of the products after the reaction.
So, mass of water formed